The present invention relates generally to plasma ashing processes for selectively removing photoresist, polymers and residues from a substrate surface. More particularly, the process relates to an oxygen-free and nitrogen-free plasma ashing process.
Ashing is a plasma mediated stripping process by which photoresist, polymer and/or residues are stripped or removed from a substrate upon exposure to the plasma. Ashing generally occurs after an etching process has been performed in which the photoresist material is used as a photomask for etching a pattern into the substrate. Additionally, the ashing process may be performed for removal of misaligned resist patterns (xe2x80x9crework wafersxe2x80x9d) and in lift-off processes. The process steps occurring prior to ashing may modify the surface of the photoresist, form polymers and/or form residues. It is highly desirable when ashing that complete removal of the photoresist, polymer and/or residues occur as quickly as possible without loss of any of the materials comprising the underlayers.
It is important to note that ashing processes significantly differ from etching processes. Although both processes may be plasma mediated, an etching process is markedly different in that the plasma chemistry is chosen to permanently transfer an image into the substrate by removing portions of the substrate surface through openings in a photoresist mask. The plasma generally includes high energy ion bombardment at low temperatures to remove portions of the substrate. Moreover, the portions of the substrate exposed to the ions are generally removed at a rate equal to or greater than the removal rate of the photoresist mask. In contrast, ashing processes generally refer to selectively removing the photoresist mask and any polymers or residues formed during etching. The ashing plasma chemistry is much less aggressive than etching chemistries and generally is chosen to remove the photoresist mask layer at a rate much greater than the removal rate of the underlying substrate. Moreover, most ashing processes heat the substrate to temperatures greater than 200xc2x0 C. to increase the plasma reactivity. Thus, etching and ashing processes are directed to removal of significantly different materials and as such, require completely different plasma chemistries and processes. Successful ashing processes are not used to permanently transfer an image into the substrate. Rather, successful ashing processes are defined by the photoresist, polymer and residue removal rates without affecting or removing layers comprising the underlying substrate.
Ashing selectivity is defined as the relative removal rate of the photoresist compared to the underlying layer. It is preferred to have an ashing selectivity of at least 50:1, wherein the photoresist is removed at least 50 times faster than the underlying substrate. More preferably, the ashing selectivity is much greater than 100:1.
During plasma ashing processes, it is important to maintain a critical dimension (CD) for the various features within a tightly controlled specification as well as promote proper underlayer surface conditions for successful metal filling in the process steps occurring after photoresist, polymer, and residue removal. Small deviations in the patterned profiles formed in the underlayers can adversely impact device performance, yield and reliability of the final integrated circuit. Traditionally, the ashing plasma has been generated from oxygen-containing gases. However, it has been found that oxygen-containing plasmas readily damage certain materials used in advanced integrated circuit manufacture. For example, oxygen-containing plasmas are known to raise the dielectric constant of low k dielectric underlayers during plasma processing. The increases in dielectric constant affects, among others, interconnect capacitance, which directly impacts device performance. Moreover, the use of oxygen-containing plasmas is generally less preferred for advanced device fabrication employing copper metal layers.
In order to overcome these problems, oxygen-free plasma chemistries have been developed. Oxygen-free plasmas can be used to effectively remove photoresist, polymers and residues from substrates containing low k dielectric materials without physically damaging the low k dielectric layer. Oxygen-free plasmas are typically generated from a hydrogen and nitrogen gas mixture that may further contain fluorine gases. However, in some cases it has been found that the use of oxygen-free plasmas containing nitrogen may alter and/or affect the chemical, mechanical and electrical properties of underlying substrate. For example, exposing carbon and/or hydrogen containing low k dielectric materials to oxygen-free plasma generated from hydrogen, nitrogen and fluorine gas mixtures results in significant damage. Occasionally, the damage is not detected during metrology inspection of the substrate after plasma processing. However, the damage can be readily demonstrated by a subsequent wet cleaning process, as may be typically employed after plasma ashing, wherein portions of the carbon and/or hydrogen-containing low k dielectric material are removed. The removed portions of the dielectric material are a source of variation in the critical dimension (CD) of the feature that is frequently unacceptable and impacts overall device yield. Moreover, even if a wet clean process is not included, the electrical and mechanical properties of the dielectric material may be changed by exposure to the oxygen-free plasmas thereby affecting operating performance. It is believed that carbon is depleted from the dielectric material during the plasma exposure. Since oxygen-free plasmas are normally generated from gas mixtures that contain nitrogen, it is believed that uptake of nitrogen occurs into the dielectric that causes problems during subsequent metal filling processes, such as the creation of voids at the bottom of trench structures.
Accordingly, it is highly desirable to have an ashing plasma chemistry that completely and rapidly removes the photoresist, polymer and residues without affecting or removing the underlying surface materials.
A plasma ashing process for selectively removing photoresist, polymers and residues from a surface of a semiconductor substrate. The photoresist, polymers and residues are removed from the substrate by exposure to a plasma that is both oxygen-free and nitrogen-free. The process includes forming reactive species by exposing a plasma gas composition to an energy source to form plasma. The plasma generated from the gas composition is free from reactive nitrogen species and reactive oxygen species. A substrate is exposed to the plasma to selectively remove the photoresist, polymers and residues from the substrate and leave the substrate substantially the same as before exposing the substrate to the plasma. The plasma ashing process may occur subsequent to an etching process, subsequent to an implantation process or during a rework or lift-off process. High ashing selectivity greater than 50:1 is achieved using the process.
The gas composition for generating the plasma consists essentially of hydrogen-bearing gas and a noble gas. Preferably, the noble gas is helium. The hydrogen-bearing gas is selected from the group consisting of hydrocarbons, hydrofluorocarbons, and hydrogen gas. The hydrogen bearing gas is in an amount ranging from about 1 percent to about 99. percent by volume of the total plasma gas composition. The gas composition may further include a fluorine-bearing gas, wherein the fluorine bearing gas is selected from the group consisting of a compound having a formula CxHyFz, wherein x ranges from 1 to 4, y ranges from 0 to 9 and z ranges from 1 to 10, HF, F2 and SF6.
In a preferred embodiment, the ashing process includes generating plasma from a gas composition consisting essentially of hydrogen gas, helium gas and tetrafluoromethane gas.
Preferably, the process is used with substrates including a carbon and/or hydrogen-containing insulating layer having a dielectric constant less than 3.0. It has been found that the dielectric constant of the carbon and/or hydrogen-containing insulating layer does not change substantially during the plasma ashing process nor does its chemical composition change. Advantageously, the process may further include a rinsing step, wherein a critical dimension of a feature in the substrate does not change substantially during rinsing. The rinsing step may include wet cleaning the substrate with an aqueous HF solution.
These and other objects, advantages and features of the invention will become better understood from the detailed description of the invention that is described in conjunction with the accompanying drawings.